Hold down the ALT key and just below the lower right corner of the ALL area click (a dashed box appears) and drag the left edge of the dashed box to the left.
To be fair, I always forget this key sequence and never achieve the size I want. Since I do a lot of scripting, I prefer to set it specifically with a script. Using your graph, click the red menu and select Save Script to Sript window. Then add a Relative Sizes() statement like the one below, and rerun the script (right click and select Run).
Here is an example using Big Class, the script is shown below the picture
Names default to here(1);
Open("$sample_data/big class.jmp");
//left frame is 120 pixels wide, which is one third the size
//of the right frame that is 360 pixels wide
Graph Builder(
Size( 511, 330 ),
Show Control Panel( 0 ),
Variables( X( :sex ), X( :age ), Y( :height ) ),
Relative Sizes( "X", [120 360] ),
Elements(
Position( 1, 1 ),
Points( X, Y, Legend( 9 ) ),
Box Plot( X, Y, Legend( 11 ) )
),
Elements(
Position( 2, 1 ),
Points( X, Y, Legend( 10 ) ),
Box Plot( X, Y, Legend( 12 ) )
)
);